Sonic Frontiers SFS-50 Tube Power Amp Reviewed

Published On: February 13, 1991
Last Updated on: October 31, 2020
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Sonic Frontiers SFS-50 Tube Power Amp Reviewed

The SFS-50 from Sonic Frontiers has a beautiful metal-on-metal aesthetic, and with 50-watts of Class A tube power it has the oomph to back up its looks. If you want to save some money back in the day, you can even build it yourself.

Sonic Frontiers SFS-50 Tube Power Amp Reviewed

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Rules are meant to be broken. This time, it's our policy of not reviewing product without UK distribution. On occasion, we make exceptions, and the Sonic Frontiers SFS-50 valve amplifier is the sort of product which qualifies as a special case.

Additional Resources
• Read audiophile power amp reviews from the likes of Krell, Mark Levinson, Audio Research, Linn, Naim, VAC, VTL, NuForce, Pass Labs and many others here.
• Read about tubes on the Audiophile blog,
• Want to read audiophile stereo preamp reviews? We have dozens from brands like ARC, Krell, Classé and many more.
• In the market for audiophile loudspeakers? Here are over 100 reviews from brands like Wilson Audio, THIEL, MartinLogan, Bowers & Wilkins, PSB, Vandersten, Magnepan and many more.

It's not as if the world is shy of decent valve amplifiers. So varied is the current selection that you can find something from under #500 on up to #20,000, Class-A, Class-B and all points inbetween, monoblocks, stereo units, four-chassis set-ups, OTLs, hybrids -- take your pick. But we just can't get enough of the utilitarian, no-frills work horses, the ones which seem to eschew the fairy dust. In other words, I'll always champion an amplifier which comes across like a latter-day Radford STA-25, Quad II or Dynaco Stereo 70.

The former lives on in the current range of Woodside descendants, the Dynaco is back in production and you can still find decent Quad IIs for sensible prices. What Sonic Frontiers offers is a mix of the good old fashioned virtues but with more grunt and more high-end applicability. So when the company offered to send one over to HFN/RR, despite the lack of a British presence, we couldn't say 'No'. And when we found out that they make 240V export versions, sell direct from the factory if you live in a country without a distributor, that the price is a mere $2195 in US funds (plus whatever it costs to ship 25kg), that they accept Mastercard and Visa and offer a five year warranty plus six months on the valves, well, we sort of owed it to you.

Sonic Frontiers, though, is not an American company. It's Canadian. Considering that the country has been making great strides in high-end manufacture over the past decade -- Classé, Oracle, Museatex and so on -- the appeal was guaranteed. Best of all, it appears that the guys at Sonic Frontiers have their hearts in the right place, admitting to being inspired by Dynaco's classic tube products. Which is why they offer their amps in kit form for a 30% saving.

Equally impressive is their choice of a 'semi' hired gun. Their collaborator is Joe Curcio, well-known to readers of the tube bible, Glass Audio, and one of the most highly-respected tube designers at work today. Among his triumphs are some modifications to the Dynaco Stereo 70, the work reflected in the design of the SFS-50. Sonic Frontiers sought his permission to incorporate some of his work and Joe lent a hand.

The SFS-50 is a tube lover's vision, all function but without any rough edges. The laboratory look starts with the satin-chrome chassis, a healthy 18in wide and 14.5in deep, standing 9in tall with the valve cage in place. The company recommends using the amplifier without the black tube cover for better ventilation; it's provided as a sop to parents and pet-owners. The transformers are finished in black with chrome caps and the sloped front panel contains an on/off switch a gold-coloured centre panel and a large meter for bias setting.

At the back are Tiffany phono inputs and chunky copper Edison Price binding posts which accept thick bare wire, spades or -- in a pinch -- banana plugs in the spaces designed for bare wire. In the area covered by the cage are two matched pairs of KT-88s (the company has been using Gold Aero tubes exclusively since September 1991), four 6DJ8s and four large capacitors. Between the banks of valves is a small rotary which selects each of the four output tubes for bias adjustment.

All this requires is a screwdriver. Switch on, leave the amp to settle in for a few minutes and dial in each valve. Next to each one is a set screw, which you turn until the needle on the meter reaches '50'. That's it.

The basic concept is one which has worked well for a number of tube revivalists, blending the tried-and-tested with the modern. The power supply is of the over-kill variety, conservatively rated and regulated 'where appropriate', with 1000uF of main filter capacitance, filter choke and custom-made shunt capacitors. The output transformers are custom-made, with a core of grain-oriented silicon steel.

Read more on Page 2

Sonic-Frontiers-SFS-40-reviewed.gifThe aforementioned valve complement consists of matched pairs for the output valves, identified for installation as the valves are packed separately for shipping. The 6DJ8s are employed in a differential cascode circuit for the input/driver stage, with a regulated B+ supply. And despite the sensible price, all of the components are designer brand luxuries, the labels including Cardas, Holco, Vishay, Wonder Cap, Rel-Cap, Wima and Solen. The wild mix of components from so many brands implies that much close listening was undertaken to choose the best make for each task.

Although an assembled unit was supplied, the company also sent along a construction manual, one which is reminiscent of the lucid guides which accompanied Dynaco amps. According to the sales blurb, a DIY-er should be able to assemble a Sonic Frontiers amp in around 10-15 hours -- not bad for a $700 savings. Assembling the amplifier also allows you to select a couple of options which, in built-up form, are restricted to dealer or distributor resetting. The review sample was configured to run in regulated screen pentode operation, with full triode operation as an option.

Pentode vs triode is one of the hot debates at the moment, but Sonic Frontiers chose to offer the choice of either, feeling that the ancillaries will determine which setting is best. Rather than tamper about inside, I merely swapped speakers until I hit on combinations which 'sang'.

The other choice to be made is for speaker impedance, which requires only minor rewiring. The review sample was set for 3.5 ohm, ideal as most of the speakers I intended to use for the review had impedances of between 3 and 6 ohms. The other settings are 8 ohms and 14.5 ohms, the latter perfect for LS3/5As and certain electrostatics.

Surprise, surprise: this 50-watter drove the Sonus Faber Extremas to more than adequate levels and worked miracles with the Apogee Stages. True, the former would sound better with a brace of SFS-50s in bi-amplified mode, but they proved an ideal test to find out about the bass capabilities of the amplifier, especially as the speaker's adjustable damping could be called on if needed. But more revealing was the way the SFS-50 worked with the Stages, convincing me even further that ribbons+tubes=nirvana...when possible.

Having lived with a Dynaco Stereo 70, in standard form and with (partial) Curcio modifications, I was expecting 'cleaned-up vintage sound'. This usually means loads of warmth but with the transparency often lacking in older designs. It's a half-way house between the rosy lushness (and flabby bass) of yore and the finely etched precision (and listener fatigue) of solid-state designs, with modern valve amplifiers somehow mixing the bext of both worlds in varying degrees. And this is what the Sonic Frontiers amp does, banishing any notion of vintage origins with a top end which exploits even the Sonus Faber tweeter.

This baby is detailed and clean, but completely devoid of 'hi-fi' edginess or spit. At first, it could be mistaken for the rolled-off filtering effect of aged glass, but it's just not the case. Transients have bite, fine details are as vivid as with the Classé DR-10 and high notes soar rather than taper. Most impressive are the dynamic contrasts in the upper frequencies, which retained their relationships even when the amp was obviously working harder than it should (through the Sonus Fabers).

I was worried that the SFS-50 wouldn't be up to the task of controlling the Stages' bass ribbons, but couldn't make them misbehave unless I turned the wick wa-a-ay up while using material with daft bottom octaves. 2 Live Crew's latest, the Professor Griff's spewings -- rap recorded by a knob-cranker proved to be too much. Back in the real world, though, and the SFS-50 sounded almost as well-controlled as the Classé. All it lacked was the final touch of extension which makes some big tranny amps sound as if they were designed to drill for oil.

Still, the SFS-50 sounded 'bigger' than it had any right to, partly because of the confident bass but primarily because of a wide and tall soundstage. Depth was only so-so, better with the Stages than any of the box speakers I tried, but that's an Apogee trait. But the up-down/left-right capabilities were consistent from speaker to speaker, so the Sonic Frontiers is a room filler in more ways than one.

Having used the amplifier for almost a month, working it hard, I'm pleased to say that it hasn't misbehaved once. Paranoids may wish to run it with the bias reduced by 10% to extend valve life, but I can only think of one area which requires special attention. This amplifier is so cable-fussy that I was going crazy trying to sort out some weird mid-band glare. Two cables stood out from the dozen or so to hand, the latest wire from Symo and Be Yamamura's A.R.T. cable, but I'm certain that others may proved usable. Just keep this amp away from single-strand solid-core, okay?

I'm tempted to call this a Stereo 70 with grunt, but it's much more than that. A hot-rodded Dynaco will match it for mid-band clarity and 'presence', and maybe at the top, but not in terms of dynamic capabilities or bottom octave resolution. I can't wait to hear how it sounds compared to the Dynaco Revived, which -- while looking like a black-clad Stereo 70 -- has benefitted from modern practices.

Can I recommend the SFS-50 to a mail-order purchaser? Am I prepared to spend your money for you, knowing that you can't hear it first unless you fly across the pond? Yes. Which is why I'm ending this review with:

Additional Resources
• Read audiophile power amp reviews from the likes of Krell, Mark Levinson, Audio Research, Linn, Naim, VAC, VTL, NuForce, Pass Labs and many others here.
• Read about tubes on the Audiophile blog,
• Want to read audiophile stereo preamp reviews? We have dozens from brands like ARC, Krell, Classé and many more.
• In the market for audiophile loudspeakers? Here are over 100 reviews from brands like Wilson Audio, THIEL, MartinLogan, Bowers & Wilkins, PSB, Vandersten, Magnepan and many more.

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